1. Field of the Invention
The invention relates to a cross-head extruder die assembly for producing fiber-reinforced thermoplastic pellets, tapes and similar products having a high fiber concentration and a process for using the same. More specifically, the invention relates to a cross-head extruder die assembly for achieving substantially the complete wetting of the filaments of multifilament strands with molten thermoplastic when producing a pultruded fiber-reinforced thermoplastic product.
2. State of the Art
Initially, pultrusion technology was limited primarily to reinforced articles produced utilizing thermosetting resins. Such products were manufactured using conventional pultrusion methods wherein resin-coated fibers are pulled through a converging heated die to form solid rods, tubes or other shapes. However, in such cases high pulling forces are generally required to move the article through the die even when low viscosity thermosetting resins are used. Thus, these methods are generally limited to the use of high strength filaments. Furthermore, the pultruded article must be gripped in some way to transmit the pulling force to the article being pulled through the die. Some articles, such as a thin wall tube, do not have sufficient strength to permit these articles to be gripped tightly enough to pull them through the die. Additionally, processing rates are generally quite slow, being in the range of 2 to 10 ft. per minute. Generally, these slow rates are due to the need to cure the thermosetting polymer by heating and the fact that the pulling force required increases rapidly with increasing speed and the increasing viscosity of the resin as the resin solidifies. Although such processes have been known for at least 15 years, they have generally not been used commercially to any great extent for the production of resin impregnated structures.
Due to the much higher viscosities of thermoplastic resins, standard pultrusion dies and techniques could not be utilized except under extremely high pulling forces, and then only at very slow speeds. U.S. Pat. No. 3,470,051 discloses the use of a thermoplastic emulsion coated roving and pultrusion equipment. In addition to being relatively slow, this method also requires a drying step which further reduces its effectiveness.
Reinforced thermoplastic materials are generally produced by compounding a reinforcing material and a thermoplastic material together on, for example, a two roll mill or in an extruder and then comminuting the crepe of extrudate thus obtained to produce granules which are subsequently molded to form the desired articles. However, cheap reinforcing fibers, particularly glass fibers are often supplied as continuous rovings consisting of several groups of filaments. Generally, it is difficult to produce satisfactory molding granules from these rovings because thermoplastic material cannot be adequately distributed throughout the roving thus leading to an unacceptable dispersion of the glass in the subsequently molded product. The present invention is directed towards overcoming this disadvantage.
Other prior art methods for preparing thermoplastic rods or tubes reinforced with glass or other fibers have deficiencies which tend to limit the quality of the products produced or the speed of the preparation. For example, U.S. Pat. No. 3,042,570 describes an apparatus and a method for preparing pellets of thermoplastic resins reinforced with glass fibers which involves passing continuous lengths of glass roving or stands into an impregnating bath of molten polymer, removing the impregnated strand bundles through various orifices, cooling the bundles, and then cutting them into lengths suitable for use in injection molding. The equipment is designed to coat the impregnated fiber bundle with a significant amount of polymer so that there is a relatively low fiber content by weight in the final product. Furthermore, the pressure on the impregnating bath is a very low pressure obtained by forcing solid plastic pellets into a melt bath using a feed screw. U.S. Pat. No. 2,877,501 describes a similar method of making glass reinforced thermoplastic injection molded pellets having a glass fiber content of 15-60% by weight. Among the potential deficiencies of these prior art methods is that the impregnation is often not complete or uniform. Additionally, high fiber contents generally cannot be obtained.
Various other processes for impregnating glass fibers and the like are further discussed in the art. For example, U.S. Pat. No. Re 32,772 discloses a compound composite structure produced by imbedding the fibers with a first resin followed by using this composite as an insert in an injection molding die so that a second resin may be molded around the composite.
U.S. Pat. No. 4,937,028 discloses forming a fiber reinforced product using a minimum of wetting resin via a two stage process. The first stage involves spreading the rovings by first passing the rovings through a bar zone (baffle) and applying force perpendicular to the width of the rovings which causes spreading of the fiber and the resultant sheet/strip is then impregnated with wetting resin and is drawn before the molding die. In a second stage, the system consists of bar zones formed by baffles substantially parallel with the first one. In this system the sheet/strip formed is impregnated with wetting resin and passes into at least two baffles opposite and parallel to each other. The system of bar zones before and after contact with the wetting resin allows for the individual impregnation of the fibers with a minimum amount of polymer.
U.S. Pat. No. 3,742,106 discloses a continuous process for producing fiber reinforced thermoplastic materials comprising passing a roving of fibers through a bed of powdered thermoplastic; separating the roving while it is in the bed; heating the roving impregnated with powdered thermoplastic to a temperature greater than the melting point of the thermoplastic resin; and passing the roving through a die while the thermoplastic is molten in order to form a fiber reinforced thermoplastic product.
U.S. Pat. Nos. 4,549,920 and 4,559,262 disclose a fiber-reinforced structure and the process for producing the structure which includes tensioning and aligning the filaments; urging the roving against a heated spreader surface to form a nip between the band of filaments and spreader surface; and heating the spreader at a temperature sufficient to maintain the polymer in the form of a melt in order to achieve complete wetting.
United Kingdom Patent No. 1302048 discloses the production of glass-filled thermoplastic polymer pellets suitable for blending with thermoplastic polymers which process comprises feeding an extruded mixture of silane coupling agent and molten thermoplastic polymer to a cross-head die, coating a continuous glass roving with said mixture in said cross-head die, solidifying said mixture while the same is coated on the roving, and subsequently pelletizing the final product into pellets suitable for use in injection molding.
U.S. Pat. No. 3,993,726 discloses a method of making reinforced plastic continuous length members by passing one or more rovings of glass or other temperature resistant fibers through a cross-head extruder to extrude and impregnate the fibers with a thermoplastic polymer.